Echo suppressor



H. NYQUIST ET AL.

ECHO SUPPRESSOR Filed Oct. 11, 1.927

uie

ramas nee. e, tera unir earner trici,-

HARRY NYQUIST, 0F MILLBURN, AND KENNETH. W. PFLEG-ER, 0F ARLINGTON, NEW JERSEY, ASSIGNOR-S 10 AMERICAN TELEPHONE AN'D TELEGRAPH COMPANY, A.

CORPORATION 0F NEW YORK.

EURO SUPPBESSOR.

Application filed. October 11, 1297. Serial No. 225,570.

a rlhis invention relates to the four-wire sections of two-way telephone systems, and'more particularly to voice-operated devices associated with the circuits for preventing echoes therein.

The four-wire sections of two-way telephone systems have one path adapted for transmission in one direction and another path adapted for transmission in the opposite direction, and these two paths are connected at each end of the circuit in the case of all-wire systems through a hybrid coil to a two-wire line, which line is approximately balanced by a suitable network. In some` systems the transmission is in part over a radio link, and in such cases the four-wire section may be connected at one end to a twowire line through a hybrid coil .and at the other end to radio apparatus for transmission `disclosure of echo suppressor arrangements as applied to all-wire circuits, reference may be had to an article by A: B. Clark and R. C. Mathes in the Journal of the American lnstitute of Electrical Engineers, June, 1925, pages 618 to 626.

If the transmission lines of the telephone system run parallel to power lines or are subject to otherA interference, the current responsive devices for disabling one path of the four-wire circuit may be falsely operated by the interferingcurrents in the voice channels.

The object of the applicants invention is to provide a voice-operated echo suppressing arrangement which is free 'from operation by one or more frequencies and their harmonics, but will respond readily to all other frequencies within the range contemplated.

ln general the applicants employ the well known amplifier-detector device for controlling a switch to short-circuit one transmission path, and balance out, or otherwise render ineffective, in the input to the amplifier, the frequencies which are to -be barred.

The novel features and the advantages of the invention will be more clearly understood when the following description of one desirable embodiment and a modification thereof is read with reference to the accompanying drawing.

Figure l of the drawing shows diagrammatically, and in association with two lines adapted for transmission in opposite direc tions, an arrangement for protecting the voice-operated apparatus against one frequency and its harmonics, the arrangement involving the use of a disk telegraphone.

Fig 2 is a view in elevation taken toward the edge of the disk, showing the disk and the associated electromagnetic devices.

Fig. 3 is a view taken toward one fiat side of the disk and shows the circumferential spacing ot the electromagnetic devices.

Fig. 4 shows diagrammatically a suitable modification of the arrangement shown in Fig. 1 for taking care, in certain cases, of two fundamental interfering frequencies and their harmonics.

Wherever like characters of reference are used in the several figures of the drawing, they designate corresponding parts.

With reference to the details of the drawing, and :with particular reference first, to Fig. 1, the telephone lines L, and L2 are understood to be the two paths of a `four-wire circuit forming part of a two-way telephone system,` as indicated and briefly discussed above. Line L, is adapted for transmission :from station Wy at the left to station E at the right, and line L2 is adapted 'for transmission in the opposite direction. It isunderstood of course 'that these voice paths are associated at one end directlyand at the other end either directly or over a radio link to a two-wire line throughthe usual hybrid coil, as clearly dis- Rlcontrolled vby the operation of the detector for short-circuiting the line L2 in response to voice currents in t e line L1. It is to be understood of course that for the suppression of echoes in the opposite direction similar apparatus is provided to short-circuit the line L1 in response to voice currents flowing in L2. If nov:r interfering currents, such as currents frompower lines, having a certain frequency, flow in the line L1, they may operate the amplifier-detector V1--V2 and the associated switchin device to produce a false short-circuiting o -the line L2.

In order to prevent the operation of the echo suppressor by currents of a. given frequency or its harmonics, 'the applicants ein ploy a device for balancing out 1n the input circuit of the amplifier V1 the given frequency and its harmonics. This device may take the form of a soft iron disk telegraphone, as will now be more speciiicallydescribed. The electromagnetic device K has its windingconnected across the line L1. This device a'nd the electromagnetic devices M and' N, which are connected series-opposing in the input circuit of the tube V1, are associated with a disk, as indicated in Fig. l and more clearly shown in Figs. 2 and 3. The device K serves to record the currents inline L1 on the disk D by .magnetizing it with varying intensity as it rotates at a speed to be discussedhereinafter. When the ma 'neti'zed portion on the disk rotates past the inductive devices M and N, voltages are induced in the windingsof M 'and N- similar in form to the line currents. The device N is located just far enough beyond the device M in the path of the rotating disk so 1 that the voltages of the fundamental interfering frequenc against which protection is desired are a w ole number of cycles out of phase in the windings of these devices, and voltages of all other fr uencies, except, of course, the harmonics o this fundamental interfering frequency, are not entirely out of phase. All harmonics of the fundamental frequency of course will, like the fundamental frequency, .be a whole number of cycles out of phase. llf the` voltages induced in M and Y N are equal, and the windings of these two devices are connected series-opposing, the resultant voltage in the input of the amplier V1, due to the interfering frequencies, is zero, as will readily be understood by those skilled in the art. Any other frequencies within the range contemplated, however, are

transmitted to the amplifier, 'their voltages in M and N not being a whole number of periods apart. rlhus it is seen that vthe arrangement for preventing operation of the amplifier-detector by a givenL frequency or its harmonics does not interfere with the operation by currents of any other frequency.

The' electromagnetic devices indicated 'in Fig. 1 as comprising an erasing device, and more clearly shown in Figs. 2 and 3 as Y and mated, then M and N may be positionedas Z, serve to demagnetize the disk after it has passed N and before .it avain passes K. The device Y may be a powerfa creases the magnet-ism to a very high value Yinl one direction, land the device Z a magnet of opposite polarity so adjusted as to bring' 'disk passes lfrom M to N be evenly divisible by the periods of the fundamental frequency to be eliminated. The spacing of M and N then must, obviously, have the proper relation to the speed of rotation of the disk. If'

a frequency of 60 cycles per second and all harmonics of that frequency are to be elimisliown in Fig. 3, and the time of one-half of a revolution of the disk must be n times seconds where n represents any whole number. ln other words, with M and N in the positions shown in the drawing, a speed of rotation of the disk of revolutions per-second isrequired to balance out the 6G-cycle fre quency and its harmonics. y

lt may be required that more than one fundamental frequency with the harmonics be eliminated and prevented from operating the echo suppressor. For instance, the line L1 may run beside 4two power lines, one carul magnet which inrying 60 cycles and another 25 cycles. ln

this case, with M and N positioned asshown, the time of onehalf of a revolution of the It m disk must be equal to E which equals 2-5 where nis one whole number and fm, is another whole number. Obviously the shortest time which satisfies both equations .is /5 second, n, being equal to 12 and m being equal to 5. 'llhus We have a required speed of 2.5l

revolutions per second.

rllie ordinary driving motor cannot run on both 60 cycles and 25 cycles. Accordingly, it

becomes more dicult in this case ofthe two fundamental frequencies to provide for the automatic adjustruent of the device to frequency changes. ln such a 'casev it becomes necessary to use two telegraphones operating in tandem, one disk being driven ley a '50 cycle synchronous motor,.for instance, and the other by a'25-cycle synchronous motor. Thisof course necessitates a` duplication of 'the ldevices K, M, N, Y, and Z, and we have lthe arrangement.diagrammatically shown in Fig. 4c of the drawing.

As indicated "above, it isimportant that the disk or disks ofthe telegrapho'ne arrange# nient be "rotated, at the proper speed. if 'the interfering,frequency can be utilized'to drive as possible.

a synchronous motor, as the actuating device for the disk, the arrangement show ndiagrammatically and in part'schematically in Fig. 1

' will take care of the case in which only one frequency with its harmonics is to be eliminated from the amplifier. .A synchronous motor arranged to drive the disk is connected across the line L1, and a higlr impedance device, a tuned circuit TC and a power amplifier are interposed. The arrangementof ig. 4 provides for two synchronous motors each connected across the line L1, as more fully disclosed in Fig. l, and arranged to drive one of the two telegraphone disks.

It should be noted that if the echo sup-' pressor is to operate with the minimum delay the recording device K should be placed on the path of the disk as close to the electromagnetic device M as is possible. If sudden changes occur i-n the load on the interfering power wires, a short period of time elapses during which M and N have induced voltages of the interfering frequencies which are unequal in magnitude and therefore will not give a-'zero resultant even though properly phased. In order that this time may be brought to the minimum, the time intervals n and m, discussedabove, should be as small It will be understood that other Btypes of telegraphone may be employed. urthermore, the electric phonographmay ,be used as a recording and reproducing device. Many other embodiments of the invention, widely diderent from that shown in the drawv ing and specifically described hereinabove for the purpose of clear illustratiom'may be used within the scope of the appended claims.

What is claimed is: 1. In association with two telephone lines ,il adapted for transmission in opposite directions, controlling means responsive to currents in one of said lines, switching means controlled thereby for disabling the opposite line, and means for preventing the operation of said controlling means by currents of a given frequency or its harmonics flowing in the first line, said last named means permitting the operation of said controlling means by currents of anypfrequency other than the given frequency or its harmonics.

2. In association with two telephoneflines adapted for transmission in opposite directions, controlling means responsive to currents in one of said lines, switching means 'controlled thereby for disabling 'the opposite line, and means for preventing the operation of said controlling means by currents of any one of a plurality of given frequencies or their harmonics owing in the first line, said .last named means permittingr` the operation of said controlling means by currents of any frequency other than the given frequencies or their harmonics.

3. In association with two telephone lines' adapted for transmission 'in opposite directions, controlling means responsive tol currents. in one of said lines, switching means controlled thereby for disabling the op Josite line, and means for balancing out in tile input of said controlling means waves of a given frequency or its harmonics.

4. In association with two telephone'lines adapted for transmission in opposite directions, controlling means responsive to currentsin one of said lines, switching means controlled thereby for disabling the opposite line, and means for balancing out in the input of said controlling means waves of any one of a plurality of given frequencies or their harmonics. j

5. In association with two telephone lines adapted for transmission inopposite directions, controlling means responsive lto currents in one of said lines, switching means controlled thereby for disabling the opposite line, two electromagnetic devices having their windings connected series-opposing in the input of said controlling means, and means for inducing in said electromagnetic devices, successively, alternating voltages corresponding to the currents in the first line, the successive actions of said means on the two electromagnetic devices being separated .by a time interval which is evenly divisible by the period of time corresponding to one cycle of a given frequency. l

6. In association with two telephone lines adapted for transmission in opposite directions, controlling means responsive to currents 1n one of said lines, switching means controlled thereby for disabling the opposite line, a 'moving member, means for recording by magnetization on said moving member the varying intensity ofthe currents in the first line, two electromagnetic devices in the path of said moving member having their windings connected series-opposing in the input ofsaid controlling means, and means for actuating said moving member, the speed at which said member .moves bearing a relation to the distance on the path thereof between said two electromagnetic devices such that the time which elapses while a point on said moving member travels between said de- 7. In assoclation withtvwA` telephone lines adapted for transmissionin opposite directions, controlling l.means responsive to cu-rrents in one of said lines, switching means controlled thereby for disabling the opposite line, a moving member, means for recording by magnetization on said moving member the varying intensityof the currents in the first lineytwo electromagnetic devices inthe path of said moving .member having their windings connected series-opposing in the irput of said controlling means, means for actuating said moving member, the speed at which said member moves bearmg a relation to the distance on the path thereof lloetvveen and means for demagnetizing said moving member.

In testimony whereof, we have signed our names to this specification this 8th day of October, 1927.

HARRY N YQUIST. KENNETH W. PFLEGER. 

